Optical disk drives, such as compact disks (CDs) and digital video disks (DVDs), are commonly used for storing large amounts of digital data on a single disc for use in audio/video or computer applications, and the like. The data on an optical disc is typically recorded as a series of "pits" arranged in tracks, where the length of the pit determines the presence of a digital "0" bit or a "1" bit. To read this recorded data, a servo system focuses a laser beam onto the surface of the disc such that the characteristics of the reflected beam allow detection of the data pits.
To this end, the servo system performs four operations: (1) a capture operation to "pull-in" the initial focus position, (2) a seek operation to move the beam radially over the surface of the disk to a desired track, (3) a centerline tracking operation to maintain the beam over the centerline of the selected track while reading the recorded data, and (4) a focus tracking operation to maintain proper will focus as the disk spins over the beam.
Conventional optical disk drives use a head assembly comprised of a laser diode for generating the laser beam which is focused onto the surface of the optical disk through an objective lens. FIG. 1 illustrates a typical optical head assembly, the operation of which is well known by those skilled in the art. A laser diode 1 produces a light beam 2 which passes through a polarization beam splitter 3 and a collimator lens (not shown). The light beam 2 is then reflected by a prism 4, through an object lens (OL) 5, and onto the surface of the optical disk (not shown). The beam 2 reflects off the optical disc, again passes through the OL 5, and then reflects off prism 4 back toward the polarization prism 3 which deflects the beam 2 onto a four-quadrant photodetector 6. The signals output by the four-quadrant photodetector 6 are used to generate a focus error signal for focusing the OL 5 and a tracking error signal for tracking the centerline of the selected track. The four-quadrant photodetector 6 also generates an RF read signal for reading the recorded data.
In order to position the read head over a selected track during a seek operation, the entire sled assembly 8 slides radially along a lead screw 9 underneath the optical disc until the read head is positioned near the desired track. This coarse positioning (or coarse seeking) is accomplished by rotating the lead screw 9 in a clockwise or counterclockwise direction. Once near the selected track, OL voice coil motors (VCMs) (10A,10B) rotate an OL carriage unit 11 about a plastic hinge 12 in a "fine seeking" operation until the OL 5 is positioned directly over the desired track. Then, as the disk rotates and the track passes under the read head, the OL VCMs (10A,10B) perform fine adjustments in a "tracking" operation in order to maintain the position of the OL 5 over the centerline of the selected track as information is read from the disc.
The OL VCMs (10A,10B) also move the OL carriage unit 11 up and down in the direction shown in order to "capture" and "track" the OL 5 focus position. For focus capture and focus tracking the four-quadrant photodetector 6 generates an astigmatic focus error signal indicative of the distance between the OL 5 and the optical disc. At the beginning of a capture operation, the OL carriage unit 11 is initially positioned sufficiently away from the disc so that it is out of focus. Then the OL VCMS (10A,10B) slowly move the OL carriage unit 11 toward the disc with the focus servo loop open until the quadrant photodetector 6 indicates that the OL 5 is within its focus pull-in range. Once within the pull-in range, the focus servo loop is closed and the initial focus point is captured. Thereafter, the OL VCMs (10A,10B) track the in-focus position in response to the astigmatic focus error signal as the read head seeks to selected tracks and reads data from the disc.
Several methods have been employed in the prior art for generating the tracking error signal used to maintain the optical transducer over the centerline of the selected track during a read operation. One method, referred to as differential phase detection (DPD), measures the phase offset between a pair of diagonal signals generated by the four-quadrant photodetector 6 to determine the position error as illustrated in FIG. 2A-2C. It should be noted that other types of photodetectors, such as a holographic photodetector, could be used to generate the diagonal signals. FIG. 2A shows three situations when the pit image is detected by the photodetector 6: left of center, at the center, and right of center. FIG. 2B shows the resulting diagonal signals generated by adding the (A+C) quadrants and the (B+D) quadrants, where the phase difference between these signals represents the position error. The position error signal (PES) is computed by converting the diagonal signals (A+C) and (B+D) into polarity square waves, as shown in FIG. 2C, and then extracting the offset or time difference between the square waves. The time difference is then integrated to generate the tracking error signal applied to the OL VCMs (10A,10B).
A problem with the above-described prior art method for generating the tracking error signal is that the differential phase error detector is dependent on the spectral content of the data being read from the disk. Thus, the randomness of the recorded data results in gain variance in the servo tracking loop; to compensate for the gain variance, the tracking servo loop is normally operated at a low (sub-optimal) bandwidth. Another drawback of prior art differential phase error detectors is a phenomenon known as "lens shift", an effective skew introduced into the diagonal signals due to generating the position error signal in continuous time.
There is, therefore, a need for an improved differential phase detector for optical disk storage devices that can generate the tracking error signal independent of the frequency content of the recorded data so that the bandwidth of the tracking servo loop can be increased to enhance performance. Another object of the present invention is to overcome the "lens shift" problem inherent in continuous time differential phase error detectors. Still another object of the present invention is to provide a differential phase error detector that can also generate a pseudo quadrature signal for use in counting track crossings during a seek operation.